1. Field of the Invention
The present invention relates generally to living olefin polymerization processes, and more specifically to initiators for such processes that are stable under reaction conditions in the absence of olefin monomer such that polymers of low polydispersity can be synthesized.
2. Discussion of the Related Art
Polymers are used in a large number of applications, and a great deal of attention has been paid to developing synthetic routes that result in polymers having optimal physical and chemical properties for a given application.
Block copolymers are one class of polymers that have broad utility. For example, block copolymers have been employed as melt processable rubbers, impact resistant thermoplastics and emulsifiers. As a result, these materials have been the focus of a particularly large amount of research and development both in industry and academia, and a variety of approaches to block copolymer synthesis have been developed.
When preparing a block copolymer, it is generally desirable to use a synthetic technique that allows for control over the chain length of each polymer block and the polydispersity of the resulting block copolymer. For some time, attempts to provide such a method have focused on block copolymer formation by living polymer synthesis. In living polymer synthesis, a metal-containing initiator having either a metal-carbon bond or a metal-hydrogen bond is reacted with an olefin monomer to form a polymer chain via the successive insertion of the first olefin monomer into a metal-carbon bond between the metal of the initiator and the growing polymer chain. If the initiator is a metal-hydride complex, the first metal-carbon bond is formed when the olefin inserts into the metal-hydride bond. When the olefin monomer is depleted, a second olefin monomer is added, and a second polymer block is formed by successively inserting, into the metal-carbon end group, the second monomer, ultimately resulting in a block copolymer including a first polymer block connected to a second polymer block. Since each polymer block is formed sequentially, the initiator and propagating species should be stable under reaction conditions in the absence of olefin monomer.
To provide a block copolymer having sizable polymer blocks of low polydispersity, the rate of chain propagation (i.e., olefin monomer insertion into the metal-carbon bond) should be substantially greater than the rate of chain termination or transfer. To prepare a block copolymer having the lowest possible polydispersity, the rate of initiation should be at least as great as the rate of propagation.
Polymerization termination is typically dominated by xcex2-hydride elimination with the products being a polymer chain having a terminal carbon-carbon double bond and the initiator having a metal-hydrogen bond. Termination of polymerization also can occur if the initiator decomposes in some other manner, such as transfer of the polymer chain from the initiator to some other element that is relatively inactive in or for olefin polymerization. Hence, the achievable chain length of copolymer blocks and the polydispersity of the block copolymer arc principally determined by the relative rates of olefin insertion and xcex2-hydride elimination, as well as initiator stability toward other modes of decomposition, especially in the absence of olefin monomer.
Attempts at synthesizing polymers using living polymer synthesis have employed a variety of initiators. For example, as reported in JACS 118, 10008 (1996). McConville and co-workers have used a diamido-titanium initiator to form polymers by polymerizing xcex1-olefins. In addition, Turner and co-workers have developed a hafnium-containing cyclopentadienyl initiator for preparing block copolymers from xcex1-olefin monomers (published PCT patent application WO 91/12285). Furthermore, Horton and co-workers report diamido-group IVB metal initiator effective in providing homopolymer synthesis (Organometallics 15, 2672 (1996)).
Despite the commercial motivation for developing a living polymer synthetic method for block copolymer preparation, known methods of block copolymer synthesis can suffer from a variety of problems. For example, the initiators used can be unstable under reaction conditions in the absence of olefin monomer, resulting in an inability to form additional homopolymer blocks to form a block copolymer. Moreover, the efficiency of block copolymer formation can be reduced due to the formation of significant amounts of homopolymer. In addition, due to the low temperatures used, the products formed using many known initiators have relatively low molecular weights and are more appropriately classified as oligomers.
As seen from the foregoing discussion, it remains a challenge in the art to provide a method of synthesizing block copolymers that includes the use of a initiator that is stable in the absence of olefin monomer such that the resulting block copolymers have low polydispersities. Such an initiator would also offer the advantage of resulting in relatively small amounts of homopolymer synthesis.
In one illustrative embodiment, the present invention provides a composition of matter having a structure:
[R1xe2x80x94Xxe2x80x94Axe2x80x94Zxe2x80x94R2]2xe2x88x92
X and Z are each group 15 atoms. R1 and R2 are each a hydrogen atom or group 14 atom-containing species. A is either
L1xe2x80x94Y1xe2x80x94L2
or 
Y1 is a group 16 atom, and Y2 is a group 15 atom. R3 is H or a group 14 atom-containing species. L1 and L2 are each dative interconnections including at least one group 14 atom bonded to Y1 or Y2.
In another illustrative embodiment, the present invention provides a method of synthesizing a block copolymer. The method comprises performing a first reaction and a second reaction. In the first reaction, a first monomeric species containing a terminal carbon-carbon double bond is exposed to an initiator containing a metal, and the terminal carbon-carbon double bonds of the first monomeric species are allowed to insert successively into the initiator to form a carbon-metal bond thereby forming a first homopolymeric block of the first monomeric species connected to the metal of the initiator. In the second reaction, a second monomeric species containing a terminal carbon-carbon double bond is exposed to the initiator, and terminal carbon-carbon double bonds of the second monomeric species are allowed to insert successively into the initiator, first inserting into the bond between the block of the first homopolymeric block and the metal of the initiator, thereby forming a copolymer including the first homopolymeric block connected to a homopolymeric block of the second monomeric species, the copolymer having a polydispersity of no more than about 1.4.
In yet another illustrative embodiment, the present invention provides a method of synthesizing a block copolymer. The method comprises: exposing a first monomeric species having a terminal carbon-carbon double bond to an initiator including a metal and allowing terminal carbon-carbon double bonds of the first species to insert successively into the initiator to form a metal-carbon bond thereby forming a first homopolymeric block of the first monomeric species having a bond to the metal of the initiator; and exposing a second monomeric species containing a terminal carbon-carbon double bond to the initiator and allowing terminal carbon-carbon double bonds of the second species to insert successively into the initiator, first inserting into the bond between the first homopolymeric block and the metal, thereby forming a copolymer including the first homopolymeric block connected to a second homopolymeric block of the second monomeric species, the method producing no more than about 25% by weight of the first homopolymer or the second homopolymer relative to a total amount of polymer product.
In a further illustrative embodiment, the present invention provides a block copolymer which comprises a first homopolymer block and a second homopolymer block connected to the first homopolymer block. The first homopolymer block comprises a polymerization product of at least about ten units of a first monomeric species having a formula H2Cxe2x95x90CHR1. The second homopolymer block comprises a polymerization product of at least about ten units of a second, different monomeric species having a formula H2Cxe2x95x90CHR2. R1 and R2 can be the same or different, and each are H or a linear, branched, or cyclic hydrocarbon that is free of non-carbon heteroatoms. The block copolymer has a polydispersity of at most about 1.4.
In still a further illustrative embodiment, the present invention provides a method of polymerization. The method comprises: reacting an initiator having a metal atom with a monomeric species having a terminal carbon-carbon double bond to allow terminal carbon-carbon double bonds of monomers to insert successively into the initiator to form a metal-capped polymer of the monomeric species connected to the metal through a metal-carbon bond. The metal-capped polymer is stable, in a solvent essentially free of the monomeric species and electron donors such as water and free oxygen at a temperature of at least about xe2x88x9250xc2x0 C. The metal-capped polymer is capable of then reacting further with monomeric species and inserting the monomeric species successively into a metal carbon bond.
In one aspect, the present invention relates to a ligand (referred to herein as [LIG]) having the following representative structures:
[R1xe2x80x94Xxe2x80x94L1xe2x80x94Y1xe2x80x94L2xe2x80x94Zxe2x80x94R2]2xe2x88x92

X and Z are group 15 atoms such as nitrogen and phosphorous that are each selected to form an anionic or covalent bond with a metal atom, particularly a transition metal, while simultaneously including two substituents (e.g., L1 and R1 or L2 and R2). Y1 is a group 16 atom such as oxygen or sulfur that is selected to form a dative bond with another atom such as a metal atom, particularly a transition metal, while simultaneously including two substituents (e.g., L1 and L2). Y2 is a group 15 atom such as nitrogen or phosphorus that is selected to form a dative bond with another atom such as a metal atom, particularly a transition metal, while simultaneously including three substituents (e.g., L1, L2 and R3).  represents a dative interconnection between X and Z, such as one or more group 14 atoms. In certain embodiments, Y1 is preferably oxygen and X and Z are the same atom, more preferably, X and Z are each nitrogen atoms.
A xe2x80x9cdative bondxe2x80x9d herein refers to a bond between a neutral atom of a ligand and a metal atom in which the neutral atom of the ligand donates an electron pair to the metal atom. As used herein, an xe2x80x9canionic bondxe2x80x9d denotes a bond between a negatively charged atom of a ligand and a metal atom in which the negatively charged atom of the ligand donates an electron pair to the metal atom.
L1 and L2 each represent a dative interconnection between X, Y1, Y2 and/or Z. L1 and L2 each correspond to at least one atom, preferably 1-4 atoms, and most preferably 2 atoms. The atoms that make up the interconnection most commonly are group 14 atoms, such as carbon or silicon. Preferably, L1 and L2 each represent a C2 unit such as xe2x80x94(CH2)2xe2x80x94, xe2x80x94(CF2)2xe2x80x94, xe2x80x94(o-C6H4)xe2x80x94, xe2x80x94CH2Si(CH3)2xe2x80x94 and the like. In certain embodiments, L1 and L2 may be selected such that X, Y1, Y2 and/or Z are not rigidly interconnected (i.e., there is at least one rotational degree of freedom between these atoms).
Although depicted in an arrangement in which X is interconnected to Y1 or Y2 and Y1 or Y2 is interconnected to Z, other arrangements of X, Y1 or Y2 and Z are envisioned to be within the scope of the present invention. For example, in certain embodiments, X may be interconnected to Z through L1 or L2. The arrangement of X, Y1 or Y2 and Z is limited only in that, simultaneously, X and Z should each be selected to form anionic or covalent bonds with a metal atom such as a transition metal while Y1 or Y2 should each be selected to form a dative bond with a metal atom such as a transition metal. Upon reading this disclosure, those of ordinary skill in the art will recognize a combination of atoms X, Y1,Y2 and Z, and interconnections L1 and L2 that will provide this capability.
R1-R3 can be the same or different and preferably are H or group 14 species such as linear, branched, cyclic and/or aromatic hydrocarbons free of non-group 14 heteroatoms that could bind to an activated metal center. One set of exemplary R1-R3 units include saturated or unsaturated straight, branched or cyclic hydrocarbons. Another example of R1-R3 units is trimethylsilyl groups. Still a further example of R1-R3 units is 2,6-disubstituted phenyl rings such as 2,6-dimethylphenyl.
In another aspect, the invention relates to metal-containing catalyst precursors, preferably group 4 metal-containing catalyst precursors, for use in the living polymerization of olefin monomers having terminal carbon-carbon double bonds. These catalyst precursors are particularly stable under reaction conditions in the absence of such olefin monomer. That is, when the reaction mixture is substantially depleted of the olefin monomer, the catalyst precursor remains stable in the absence of water, oxygen, basic donor ligands and the like. As a result of the catalyst precursor""s stability, the resulting polymers (e.g., homopolymers, random copolymers and/or block copolymers) have low polydispersities. Furthermore, when used to prepare block copolymers, the amount of homopolymer produced is relatively low.
Substantial depletion of an olefin monomer relates to a situation in which the olefin monomer is present in an amount below the detection limit of standard NMR spectrometers such that the olefin monomer cannot be detected using such standard NMR spectrometers. Typically, an olefin monomer is substantially depleted when less than about 5% of the olefin monomer remains as olefin monomer in solution relative to the amount of olefin monomer initially present in the solution.
The catalyst precursors of the present invention have the following representative molecular structures:
[R1xe2x80x94Xxe2x80x94L1xe2x80x94Y1xe2x80x94L2xe2x80x94Zxe2x80x94R2]MR4R5
[Xxe2x80x94L1xe2x80x94Y1xe2x80x94L2xe2x80x94Z]MR4R5

M is a metal atom that can form a metal-carbon bond into which an olefin can be inserted. Those of ordinary skill in the art will recognize metals that meet this requirement. For example, M may be selected from metals of groups 3-6, late transition metals such as those of group 10, actinides and lanthanides. In one set of preferred embodiments, M is selected from Ti, Zr or Hf. X and Z each form an anionic or covalent bond to M while Y1 or Y2 each form dative bonds to M. Preferably, the length of the Mxe2x80x94Y1 and Mxe2x80x94Y2 bonds is at most about 2.5 Angstroms, more preferably at most about 2.3 Angstroms, most preferably at most about 2.1 Angstroms, depending upon the size of M.
R4 and R5 should be good leaving groups such that living polymerization can occur via the removal of R4 or R5 and the formation of an initiator, as described below. Typically, R4 and R5 are substantially similar to R1-R3. Preferably, R4 and R5 are linear or branched alkyls having a length of from 1-10 carbon atoms. In some embodiments R4 and/or R5 can be hydrogen.
The present invention is not limited by the particular geometrical configuration of the catalyst precursor. However, in certain embodiments, the catalyst precursor may have a nonplanar geometry, such as, for example, trigonal bipyramidal. In some embodiments, it is preferable that the catalyst precursor have a geometrical configuration such that X, Y1 or Y2 and Z are interconnected in the same plane.
In a particularly preferred set of embodiments, a catalyst precursor is provided having one of the structures: 
It is to be noted that, in certain embodiments, any or all of the isopropyl groups of [(2,6-i-Pr2xe2x80x94C6H3NCH2CH2)2O]M(R4)(R5) may be replaced with H or branched or straight chain alkyl groups. As will be appreciated by one skilled in the art, such alkyl groups should be selected such that an olefin monomer""s access to M during polymerization (described below) is not sterically hindered by these alkyl groups. Typically, such alkyl groups have at most about 20 carbon atoms and include, for example, methyl, propyl, t-butyl and the like.
The catalyst precursors can be prepared using standard alkylation techniques. For example, the protanated ligand (H2[LIG]) can be reacted with M(NMe2)4 to form [LIG]M(NMe2)2 which is then reacted with TMSCl to form [LIG]MCl2. The [LIG]MCl2 is reacted with Rxe2x80x94MgX to provide [LIG]MR2. The appropriate reaction conditions of from about xe2x88x9278xc2x0 C. to about 0xc2x0 C. in a solvent such as ether, diethyl ether, hydrocarbons, free of oxygen and water, can be selected by those of skill in the art. Alternatively, [LIG]MCl2 can be reacted with aluminoxane which first reacts to form the dimethyl compound [LIG]M(Me)2 in situ, and then removes one Me group to make the active cation, serving as its counterion. This reaction is known, as described in, for example, published PCT patent application WO 92/12162.
During living polymerization, the catalyst precursor is activated via the removal either R4 or R5, typically in situ, to form an initiator which is cationic in nature. Where a stable salt can be synthesized, this salt can be provided, stored, and used directly. Counterions for the initiator should be weakly-coordinating anions, for example [B(C6F5)4]xe2x88x92. Those of ordinary skill in the art can select suitable counter ions.
The initiator can be reacted with monomeric olefins having a terminal carbon-carbon double bond (H2Cxe2x95x90CHR6) to provide polymers, where R6 is hydrogen or a hydrocarbon such that the olefin can be a straight, branched, cyclic or aromatic hydrocarbon. Furthermore, the hydrocarbons may include additional carbon-carbon double bonds. Preferably, any additional carbon-carbon double bonds are internal (non-terminal). Preferably, these monomers are substantially devoid of any heteroatoms. Examples of such monomers include, but are not limited to, xcex1-olefins such as ethylene, 1-propylene, 1-butene, 1-hexene, 1-heptene, 1-octene, 1-nonene, 1-decene, 4-methyl-1-pentene and the like.
Initiation of the polymerization reaction occurs by insertion of the carbon-carbon double bond of the species H2Cxe2x95x90CHR6 into a metal-carbon bond of the initiator. During reaction of the initiator and monomeric olefin, chain growth of the polymer occurs by successive insertion of the monomer into a bond formed between the terminal carbon atom of the polymer chain and the metal atom of the initiator. It is an advantageous feature of the present invention that, under reaction conditions in the absence of monomer (described above), such a metal-carbon bond remains stable for periods of time sufficient to allow depletion of monomer and subsequent addition of monomer and continued chain growth. For example, the system allows depletion of one monomer H2Cxe2x95x90CHR6, and addition to the system of a additional monomer H2Cxe2x95x90CHR7 that can be the same monomer (for continued homopolymer growth) or a different monomer (for block copolymer synthesis). Preferably, a metal-carbon bond of the initiator, such as a bond between the metal and a polymer chain, remains stable for greater than about a half an hour at room temperature under reaction conditions in the absence of olefin monomer, water, oxygen, basic donor ligands or the like. For most known initiators used in polymerizing these monomers, the metal-carbon bond formed between the initiator and the polymer chain is not stable enough for standard analytical techniques, such as NMR, to verify the existance of the initiator, indicating that the initiator-polymer chain species is not stable for more than at most about one second at room temperature. In contrast, the initiating and propagating species of the present invention have been verified by NMR.
This enhanced stability of this metal-carbon bond is desirable because blocks of polymer may be formed in a sequential fashion by adding olefin monomer, allowing the olefin monomer to react until it is depleted and subsequently adding more olefin monomer. When forming block copolymers, a first block of the copolymer may be formed (first homopolymeric block). Upon depletion of the first monomeric olefin, the carbon-metal bond remains stable and a second olefin monomer may be added to the reaction mixture to form a second homopolymeric block that is connected to the first homopolymeric block. During this reaction, the second olefin monomer first inserts into the metal-carbon bond formed between the first homopolymeric block and the initiator. Subsequently, the second olefin monomer successively inserts into the metal-carbon bond formed between the initiator and the polymer chain of the second olefin monomer.
As a result of the initiator""s stability, polymers are formed with relatively low polydispersities. The xe2x80x9cpolydispersityxe2x80x9d of a polymer as used herein refers to the ratio of the weight average molecular weight (Mw) to the number average molecular weight (Mn) of the polymer according to equation 1.                    POLYDISPERSITY        =                                            ∑              i                        ⁢                                          N                i                            ⁢                              M                i                2                                                                        ∑              i                        ⁢                                          N                i                            ⁢                              M                i                                                                        (        1        )            
where Ni is the number of mer units having molecular weight Mi.
In particular, the present invention can provide block copolymers having low polydispersities. Known block copolymers have been synthesized using anionic polymerization processes, but xcex1-olefin monomers cannot be used in these processes. In known block copolymers, typical minimal polydispersities are on the order of about 1.5. According to the present invention, block copolymers preferably have a polydispersity of at most about 1.4, more preferably from about 1 to about 1.3, more preferably from about 1 to about 1.2, more preferably from about 1 to about 1.1, and most preferably from about 1 to 1.05. The polydispersity of a polymer can be measured directly by a variety of techniques including, for example, gel permeation chromatography or by standard tests such as the ASTM D-1238 procedure.
It is a further advantage of the present invention that the initiator""s stability results in good block copolymer formation with minimal formation of polymers formed substantially only of individual monomeric olefin units (homopolymer). That is, relatively highly pure block copolymer is formed. In known systems, the amount of homopolymer formed is typically about 30 wt % based on the total amount of polymer formed including the block copolymer. According to the present invention, the amount of homopolymer formed is at most about 25 wt % based on the total amount of polymer formed including copolymer, more preferably at most about 15 wt %, and most preferably at most about 5 wt %. These purity levels are preferably realized in combination with preferred polydispersity levels discussed above. For example, one embodiment involves formation of block copolymer of polydispersity of less than about 1.4 with homopolymer formation of at most about 25 wt % based on the total amount of polymer formed including copolymer.
Most known block copolymer synthesis methods are conducted at temperatures of at most about xe2x88x9278xc2x0 C. At these low temperatures, it is difficult to form polymers. Instead, oligomers having less than 50 mer units typically are formed. It is a further advantage of the present invention that living polymerization processes can be successfully conducted at relatively high temperatures. Preferably, living polymerization occurs at a temperature of at least about xe2x88x9250xc2x0 C., more preferably at least about 0xc2x0 C., most preferably at least about 25xc2x0 C. At these higher temperatures in connection with the present invention, polymer blocks having at least about 50 mer units, preferably at least about 75 mer units, and most preferably at least about 100 mer units can be formed.
The initiators of the present invention can be used for polymerization of a variety of combinations of monomers to form homopolymers, random copolymers of any number or ratio of monomers, or block copolymers of any number and size of blocks, while providing optionally the preferred polydispersities and/or purities discussed above. For example, two monomers A and B (H2Cxe2x95x90CHR6 and H2Cxe2x95x90CHR7) in a ratio of 2:1 can first be provided in a reaction system, with polymerization resulting in a random copolymer with A and B being incorporated in a ratio of 2:1, after depletion of these monomers. Then, because of the stability of the initiator, additional monomers C and D can be added to the system, and further polymerization will result in a product having a first block of random AB and a second block of random CD. As discussed, blocks of relatively pure homopolymer can be provided. For example, polymerization of A until depletion of A, followed by addition of B and polymerization of B resulting in a block copolymer AB.
The following examples indicate certain embodiments of the present invention. These examples are illustrative only and should not be construed as limiting.
All air sensitive manipulations were conducted under a nitrogen atmosphere in a Vacuum Atmospheres drybox or under argon when using Schlenk techniques. Pentane was washed with sulfuric/nitric acid (95/5 v/v), sodium bicarbonate, and then water, stored over calcium chloride, and then distilled from sodium benzophenone ketyl under N2. Reagent grade diethyl ether, 1,2-dimethoxyethane, 1,4-dioxane, and tetrahydrofuran were distilled from sodium. Deuterated solvents were passed through activated alumina and vacuum transferred to solvent storage flasks until use. Proton and carbon spectra were referenced using the partially deuterated solvent as an internal reference. Fluorine spectra were referenced externally. Chemical shifts are reported in ppm and coupling constants are in hertz. All spectra were acquired at about 22xc2x0 C. unless otherwise noted. IR spectra were recorded on a Perkin-Elmer FT-IR 16 spectrometer as Nujol mulls between KBr plates in an airtight cell. Microanalyses (C, H, N) were performed on a Perkin-Elmer PE2400 microanalyzer in our laboratory. Since the elemental analyzer measures moles of water, the % H was calculated assuming all D present was H, but the actual molecular mass was employed. GPC analyses were carried out on a system equipped with two Alltech columns (Jordi-Gell DVB mixed bed xe2x88x92250 mmxc3x9710 mm (i.d.)). The solvent was supplied at a flow rate of 1.0 mL/min. with a Knauer HPLC pump 64. HPLC grade CH2Cl2 was continuously dried and distilled from CaH2. A Wyatt Technology mini Dawn light scattering detector coupled to a Knauer differential-refractometer was employed. The differential refractive index increment, dn/dc, was determined assuming that all polymer that was weighed for the run (usually about 5 mg to xc2x10.1 mg) eluted from the column. For poly(1-hexene) polymers, to minimize polymer weighing error the average value for dn/dc (0.049 mL/g) from 18 runs (0.045 to 0.053 mL/g) was employed and the molecular weights recalculated. The yields for poly(1-hexene) were essentially quantitative (about 97% to about 100%).